Shift work effects millions of people and many experience serious health-related consequences, such as shortened and disrupted sleep, fatigue, impaired performance, and gastrointestinal disturbances. Night shift workers experience these symptoms because their internal circadian rhythms rarely phase shift to align with the new (daytime) sleep schedule. One reason it is so difficult for these individuals to shift is that they continue to be exposed to the natural 24-hour light-dark cycle, a very powerful zeitgeber that works to keep rhythms synchronized for alertness during the day and sleep at night. One strategy for shift work adaptation is to force the circadian pacemaker (clock) to phase shift to realign with shifted sleep-wake schedules, while another approach is to try to improve shift worker's daytime sleep and nighttime performance without shifting rhythms. Melatonin is a hormone normally secreted by the pineal gland at night. Previous studies suggest that exogenously-administered melatonin has phase-shifting as well as sedative effects. In Study 1, the investigator will determine whether appropriately-timed melatonin can phase shift the circadian pacemaker to align with a 9-hour shift of the sleep-wake schedule in subjects who do simulated night work and are exposed to the natural 24-hour light-dark cycle and other zeitgebers. Phase shifts will be assessed using phase measures derived from endogenous melatonin secretion and core body temperature. In Study 2, the investigator will investigate the immediate effects of two different doses of melatonin on daytime sleep after simulated night work, and assess the effects of melatonin treatment on nighttime performance. Sleep will be polygraphically-recorded to assess changes to sleep architecture, and multiple sleep latency tests and computerized performance tests will be performed during the night shifts to assess alertness.